Microporous films, sheets, and membranes are materials that have structures which enable fluids to pass readily through them. These materials have pores whose effective size typically is at least several times the mean free path of the flowing molecules, namely from several micrometers down to as low as about 100 Angstroms. Sheets made from the materials generally are opaque, even when made from an originally transparent material, because the surfaces and internal structure scatter visible light.
Microporous membranes enjoy utility in a wide range of divergent applications, including use in the filtration of solid materials, ultrafiltration of colloidal matter, use as diffusion barriers or separators in electrochemical cells and uses in the preparation of synthetic leathers and fabric laminates. The latter requires the membranes to be permeable to water vapor but substantially impermeable to liquid water when used to prepare such articles as shoes, raincoats outer wear, camping equipment, and the like. Microporous membranes also are utilized in the filtration of antibiotics, beers, oils, bacteriological broths, and for the analysis of air, microbiological samples, intravenous fluids and vaccines. Surgical dressings, bandages and other fluid transmissive medical articles likewise incorporate microporous membranes and films. Microporous membranes also are commonly employed as battery separators.
For more particularized applications microporous membranes may be laminated onto other articles to make laminates of specialized utility. Such laminates may include, for example, a microporous layer laminated to an outer shell layer to make a particularly useful garment material. Microporous membranes may also be utilized as a tape backing to provide such products as vapor transmissive wound dressing or hair setting tapes and the like.
A number of methods for making microporous films and membranes are taught in the art. One of the most useful methods involves thermally induced phase separation. Generally such a process is based on the use of a polymer that is soluble in a diluent at an elevated temperature but that is insoluble in the diluent material at a relatively lower temperature. The so-called "phase transition" can involve a solid-liquid phase separation, a liquid-liquid phase separation or a liquid to gel phase transition. Examples of such methods are described in U.S. Pat. Nos. 4,247,498, 4,539,256, 4,726,989, and 4,867,881.
Typically, state-of-the-art processes that employ normally melt-processable polymers produce films and membranes with relatively low puncture resistance. To overcome this limitation for applications where mechanical strength and puncture resistance are desirable, a component of an ultra-high molecular weight polyolefin typically is added to the film or membrane to boost its mechanical integrity and puncture strength. U.S. Pat. No. 5,051,183 (Takita et al.), for example, describes making microporous films having at least one percent by weight of an ultra-high molecular weight polyolefin. While the addition of ultra-high molecular weight materials can favorably address problems of mechanical integrity, blends containing these additions are not normally melt-processable and must further incorporate plasticizers to become melt-processable. Thus use of blends containing such additions generally adds complexity and cost to processing techniques.